Calibration of lasers that produce multiple power output levels of emitted radiation

ABSTRACT

A laser calibration apparatus and method first calibrates a maximum laser power level. Then lesser laser power levels are calibrated. A separate calibration pattern is used for calibrating each of the laser power levels. The calibration patterns used for the lesser laser power levels include the calibrated maximum laser power level. Such inclusion established a fixed relationship between the maximum laser power level and all of the lesser laser power levels. The calibration is performed on an optical disk, preferably of the magneto optical type. The calibrated laser power levels are then used to record pulse-width modulated signals.

RELATED APPLICATION

This application is a division of Ser. No. 08/612,994, filed Mar. 8,1996, now U.S. Pat. No. 5,617,401.

FIELD OF THE INVENTION

This application relates to calibrating lasers for producing a laserbeam having a succession of a plurality of diverse laser beamintensities, particularly those lasers for producing a rapid successionof said plurality of diverse beam intensities.

BACKGROUND OF THE INVENTION

Lasers have enjoyed a wide range of practical applications. It isdesired to provide laser operations that provide a succession of steppeddiverse laser beam intensities. A laser beam having a succession ofstepped diverse beam intensities may find a wide divergence ofapplications. For illustrating the present invention, an application ofthe invention to calibrating a laser for recording data on amagneto-optical disk is shown and described. In the application tomagneto-optical disks, a rapid succession of diverse laser beamintensities is desired.

In recording of any kind, including recording on optical disks, it is acontinuing desire to increase the areal density of the recording. Oneway to increase such areal density is to increase lineal recordingdensity along tracks on an optical disk. Earlier recording techniquesemployed magnetic transitions for recording and indicating binaryinformation. Sensing these transitions produce output pulses indicatingthe recorded binary information. To successfully record binaryinformation, all such magnetic transitions should be in a so-calledtransition position, also called a cell. Such data recording is termedpulse-position modulation (PPM). PPM can take many forms such asnon-return-to-zero change on one's recording (NRZI). Non data or clocktransitions were then added to NRZI recording to produce the well-knownphase-encoding and frequency-modulation recording. In all of theserecordings binary data are indicated by data-indicating magnetictransitions using known rules. Later, rather than representing user datavalues, such indications indicate 1's and 0's of a recording code, suchas a d,k code. Even with all of these advances in the recording art,intersymbol interference (ISI) in PPM recording tends to limit thelinear recording density of optical disks.

Pulse-width modulation (PWM) is desired to enhance lineal recordingdensity over prior PPM recording techniques. Each data cell in PWMincludes a mark and a gap. Duration or width of a mark indicates a blockof information, such as one d,k code block. A leading recordedtransition (herein arbitrarily indicated as a positive-going transitionP) indicates an onset or leading edge of a mark that is usually also anonset of a new data cell. A mark-trailing or negative-going transition Nis within a data cell and indicates end or edge of a mark and onset of agap. Herein such leading and trailing transitions are respectively andarbitrarily shown as positive and negative going transitions P and N. Toobtain high linear recording densities, higher than PPM densities, suchPWM data cells are extremely short. As such, PWM introduces a need foran enhanced recording system. One such enhanced PWM recording system isshown in Belser et al U.S. Pat. No. 5,400,313. In this optical diskrecording/reading system, a minimal number of circular marks on theoptical disk record a desired run length of coding to be recorded. Apreferred coding is a known d,k code having 1,7 parameters. Thisrecording system provides for accurately locating mark-gap transitionson the optical disk, a necessity for high-density PWM recording. Torecord a mark, a plurality of short laser pulses having selectedrecording power levels are used. Belser et al show three laser pulsepower levels that may be combined for recording the range of marksallowed by a recording code. The recording pulse power levels areselected based on recording patterns for accurately obtaining sharprecorded edges or transitions. Such laser pulse recording levels shapethe mark on the recording layer for enabling more faithful reproductionof recorded data. Co-pending commonly-assigned application by Hurst,Jr., Ser. No. 08/342,196, filed Nov. 18, 1994, teaches amulti-power-levels for a mark in a recording system in which a novelpre-heating operation is employed for obtaining enhanced recording.

Since optical disks are usually removable and made by several vendors,optical disk recording parameters vary between disks such that aseparate calibration is needed for each optical disk. Therefore, it isdesirable to calibrate the laser for writing on each disk as it isreceived into (mounted) an optical disk drive. Usually such mounting iscommanded by an attaching host processor. It is important to ensure thatthe attaching host processor does not wait very long before accessing amounted optical disk. Accordingly, write calibration should beaccomplished in a minimal time. In accordance with this invention, acalibration procedure is provided that enables one calibration to effectcalibrations of a plurality of recording power levels.

DISCUSSION OF PRIOR ART

Romeas et al, in U.S. Pat. No. 4,631,713, show recording binary testwords on a write-once optical disk having a 1-0 monotonous repeatedpattern to calibrate a laser for PPM recording. The durations of therespective "1" and "0" portions of the test pattern are measured. Thelaser write power that results in equality of the durations of the 1 and0 portions is selected as the recording value.

Bletscher, Jr. et al in U.S. Pat. No. 5,070,495 show an extensivecalibration system for PPM recording based on symmetrical parameters incalibration patterns.

Call et al in U.S. Pat. No. 5,185,734 shows calibrating a DAC (digitalto analog converter) to supply a given output analog signal for anyinput digital value within a range of input digital values. Thiscalibration means that input digital values can be used to activate acalibrated DAC to produce accurate analog laser-drive signals to diversepower levels.

Call et al in U.S. Pat. No. 5,216,659 show a laser power calibration bymeasuring laser drive current in an out-of-focus beam condition at thesurface of a WORM medium and in an in-focus condition of the laser beamat the disk surface. A slope is generated representing a variation inlaser beam power level versus laser current that enables calculations oflaser power based on laser current. A first step calibrates the laser inan out-of-focus condition. A second step completes the calibration in anin-focus laser beam condition.

SUMMARY OF THE INVENTION

The present invention provides for calibrating a recording system forhigh-linear-density pulse-width recording.

A first one of a plurality of laser pulse power levels is calibratedusing a recorded calibration pattern in which the first one of theplurality of laser pulse power levels is varied in power level. Then oneor more intermediate laser power levels are calibration using thecalibrated first one of the laser pulse power levels at the calibratedlevel while varying power levels of one or more of the intermediatelaser pulse power levels.

A least squares calibration procedure identifies an optimum laser pulselevel for each of the plurality of laser pulse power levels. In anoptical disk, a plurality of calibration sectors repeatedly receive acalibration pattern for calibrating a laser pulse power level with thelaser pulse power level to be calibrated varying from sector to sector.Each laser pulse power level to be calibrated has a unique lasercalibration pattern consisting of marks and gaps. Marks are recordedwith varying recording powers for each laser pulse level to becalibrated while other power levels are held constant in all sectors.

Digital signal processing circuits process calibration signals read fromthe calibration sectors to produce calibration data that includesfinding an average mark duration, average gap duration, number of marksand gaps, time clock period duration derived from the averaged mark andgap durations, desired mark and gap durations. A least squares analysisidentifies optimum laser power level for each recording power level.Linear interpolation between two sectors having calibration data closeto optimum values accurately identifies the optimum laser power level.

A specific embodiment calibrates three recording laser pulse powerlevels to be used with at least one uncalibrated laser pulse power levelused in recording marks and gaps for pulse width modulated signals.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

DESCRIPTION OF THE DRAWING

FIG. 1 illustrates in simplified block diagram form a data storagesystem in which the present invention is advantageously employed.

FIG. 2 is a simplified block diagram of calibration circuits used in theFIG. 1 illustrated data storage system.

FIG. 3 diagrammatically illustrates a portion of a pulse-width-modulatedsignal.

FIGS. 4 and 5 are simplified flow charts showing an optical drive lasercalibration of the FIG. 1 illustrated data storage system.

DETAILED DESCRIPTION

Referring now more particularly to the appended drawing, like numeralsindicate like parts and structural features in the various figures. Anoptical data storage system shown in FIG. 1 incorporates an illustrativeembodiment of the present invention. Host 10, a programmed computer,attaches optical disk system 12. Disk system 12 includes adaptor 14 thatconnects to host 10 for exchanging control and data signals therewith.Programmed control 16 that includes a programmable processor (notshown), has program storage 18 for storing program data to operate disksystem 12. Laser control 20 includes those circuits that operate alaser, including those circuits for generating laser beam power signals.Optical system 22 has the usual circuits and optical elements used tocontrol an optical disk drive 30. Laser driver 24 receives controlsignals from laser control 20 for operating laser 26. Laser 26 emits abeam of radiation to optics 28 that focusses and directs the beam ofradiation to optical disk 32 over light path 31. Optical disk drive 30includes a carriage (not shown) radially movable with respect to opticaldisk 32 for enabling scanning a spiral track (not shown) on optical disk32. Detector 34 receives a reflected beam from optical disk 32 viaoptics 28, as is known. Detector 34 includes the usual data detector,servo detector and the like. Data flow 38 processes data and controlsignals that pass between adaptor 14 and optical system 22 for recordingon and reading from optical disk 32. Calibration circuits 40 areconstructed in accordance with the present invention for providingenhanced calibration for use in pulse-width-modulated (PWM) recordedsignal 39 (FIG. 3). The unnumbered double-headed arrows in FIG. 1represent usual electrical connections between the illustratedcomponents. Double-headed arrow 41 represents later-described signallines that carry calibration-related data and control signals betweencalibration circuits 40 and programmed control 16.

This description is pointed to calibrating the FIG. 1 illustratedoptical drive 30 for operating with one optical disk. If optical system12 attaches a plurality of optical disk drives 30, then calibrationcircuits 40 (FIG. 2) are used in connection with any one of the opticaldisk drives, one at a time. Calibration circuits 40 embody thecalibration methodology of this invention as later set forth withrespect to FIGS. 4 and 5. Calibration circuits 40 determine durations ofthe marks 43 and gaps 45 (FIG. 3), sum the determined durations and thenumber of marks and gaps. Upon completing one calibration procedure,calibration circuits 40 supply the generated calibration data toprogrammed control 16 for calculating optimum laser power for the laserpower level being calibrated.

The following description pertains to processing one sector of acalibration pattern, it being understood that many calibrating sectorsare involved in each calibration procedure. In a first calibratingoperation, calibration circuits 40, line 42 carries analog read signalsfrom detector 34 to an analog-to-digital convertor (ADC) 44 forconversion into a sequence of multi-digit digital signals, hereafterdigital signals. The sequence of digital signals represents theamplitudes of the read signals with respect to time. Digital equalizer46 processes digital signal output of ADC 44 to supply a sequence ofequalized digital signals to edge detector 48 and threshold detector 50.Clock recovery circuit 52 is connected to edge detector 48 and thresholdgenerator 50 in a read back clock servo loop, i.e., generate a clock ortiming signal based upon detecting transitions recorded on optical disk32. Edge detector 48 supplies to mark-gap calculator 62 a sequence ofdetected transition-time indicating digital signals over multi-line bus58. A transition-time indicates time of occurrence of a transitionrecorded on an optical disk, such as a magnetic transition in amagneto-optical disk. A sign P/N (indicates positive transition P ornegative transition N) signal on line 60 indicates transition-timepolarity associated with each multi-digit digital representation of thetransition-time. Mark-gap calculator 62, for distinguishing betweenmarks and gaps, calculates the difference between two successivedigitally-indicated transition-times. As shown in FIG. 3, a read mark isindicated by a digitally-indicated leading positive (P) transition-timederived from a recorded mark-leading transition followed by adigitally-indicated trailing negative (N) transition-time derived from arecorded mark-trailing transition. A read gap is correspondinglyindicated by a digitally-indicated trailing negative (N) transition-timederived from a recorded gap-leading transition followed by adigitally-indicated trailing positive (P) transition-time derived from arecorded gap-trailing transition. The digitally indicatedtransition-times are digitally amplitude qualified for rejecting noise.

It is to be remembered that a magnetic polarity transition on opticaldisk 32 of the calibration pattern results in an analog transition inthe read signals. The digital values in the digital signals indicateanalog read signal amplitude. For example, a presence of a magneticdomain recorded on disk 20 results in a maximum signal amplitude whileabsence of a magnetic domain is indicated by a minimum signal amplitude.This relationship of signal amplitude to the presence or absence ofmagnetic domains is based solely on circuit design. Amplitudequalification of the pulses is achieved by comparing the receiveddigital signal values with a predetermined amplitude threshold value. Apredetermined number of successive digital signals having values greaterthan the threshold value indicate the presence of a magnetic domain. Thepredetermined number is empirically determined to represent an amplitudethat excludes noise.

Mark-gap calculator 62 orients its calculation based upon a sign forcalculating duration of each mark, herein arbitrarily defined as elapsedtime between a leading positive (P) transition-time and a trailingnegative (N) transition-time. Similarly, gap durations are measured aselapsed time between a leading negative transition-time and a trailingpositive transition-time.

Upon detecting either a mark or a gap, mark-gap calculator 62 sends aduration indicating digital signal over bus 64 to mark-gap qualifier 66.Simultaneously to the bus 64 signal, a mark (P) indicating signal online 68 indicates a mark while a gap (N) indicating signal on line 68indicates a gap. Marks alternate with gap indications. Mark-gapqualifier 66, timed by clock recovery circuit 52, measures durations ofmarks and gaps to ensure that each mark has a greater duration than apredetermined minimum duration but not exceeding a predetermined maximumduration. Gaps are duration qualified in the same manner. The minimumsand maximums for marks and gaps are design choices.

Synchronously to the first operation described above, second operationsof calibration circuits 40 accumulate calibration data for forwarding toprogrammed control 16. The accumulated calibration data are forwarded toprogrammed control 16 at the completion of circuits 40 calibrationoperations in each calibration sector.

Four register R0 80, R1 81, R2 82 and R3 83 accumulate the calibrationdata. The letters "M" and "G" in the registers respectively indicatethat mark and gap calibration data are stored in the registers.Registers R0 80 and R1 81 respectively accumulate the total of mark andgap measured durations. Similarly, counting registers R2 82 and R3 83respectively total the number of marks and gaps detected while readingthe recorded calibration pattern. Reset line 84 (part of lines 41 inFIG. 1) carries a reset signal received from programmed control to resetregisters R0-R3 to a cleared state. Accumulation of the calibrationinformation is timed and sequenced by mark-gap qualifier 66. Uponcompleting qualifying any mark or gap, mark-gap qualifier 66 sends agap/mark indicating signal over line 88 to registers R0-R3 to selectregisters R0 and R2 to accumulate mark information or registers R1 andR3 to accumulate gap information. Registers R2 and R3 respondrespectively to the gap/mark indicating a mark or a gap to tally thenumber of marks and gaps that have been measured. Similarly, register R0and R2 accumulate measured durations or widths of measured marks andgaps. A mark indicating signal on line 88 activates register RO to anactive condition and deactivates register R1. A gap indicating signal online 88 activates register R1 to the active condition and deactivatesregister R0. Duration accumulation is achieved by mark-gap qualifiersending a measured duration to sum calculator 92. Sum calculator has amulti-digit adding register adder (not shown) for storing the receivedmeasured duration. Sum calculator 92 responds to receipt of the measuredduration to read the active register R0 or R1 and add the contents of R0or R1 to the just-received measured duration. The sum is then returnedto the respective register R0 or R1 for accumulating all of the measureddurations respectively for marks and gaps. The above-describedoperations are repeated until all calibration sectors have been read.

Upon programmed control 16 detecting completion of reading each sector,it sequentially reads the contents of registers R0-R3 for calculatingadditional later-described calibration parameter data. Programmedcontrol 16 supplies a register select signal on line 96 to activateserializer 98 to sequential read registers R0-R3, then serializes theread accumulated calibration data for transfer over line 100 toprogrammed control 16. Lines 96 and 100 are represented in FIG. 1 bynumeral 41. Upon reading all four registers R0-R3, programmed control 16resets registers R0-R3 for any ensuing calibration.

For generating calibration parameter data, programmed controller 16calculates and stores, for each calibration sector, an average markduration and an average gap duration based upon later describedcalibration signal patterns. Equations 1-6 below set forth theprogrammed control 16 calculations:

The average mark duration <mark> is calculated as: ##EQU1##

In Equation 1, L indicates the "ith" measured mark "m" duration, N isthe number of marks read from the sector while R₀ /R₁ indicates that thecontent of register R0 is divided by the content of register R1.

The average gap duration is calculated as: ##EQU2##

In Equation 2, L indicates the "ith" measured gap "g" duration, N is thenumber of gaps read from the sector while R₁ /R₃ indicates that thecontent of register R1 is divided by the content of register R3.

The variables R₀ through R₃, the contents of registers 80-83,respectively, are calculated as set forth below in equations 3 through6. The other constants for each calibration pattern are defined inequations 3 through 6. ##EQU3##

In equation 3, N_(m) is the number of marks, L is the "ith" measuredmark m duration with the summation being stored in register R0 as valueR₀.

Register R1 81 contains the sum of all the gap durations in thecalibration pattern. That sum is calculated as set forth in equation 4:##EQU4##

In equation 4, N_(g) is the number of measured gap g durations in thecalibration pattern and "L" is the "ith" gap duration in the sector.

Equation 5 shows that register R2 82 contains the number of measuredmark durations N_(m), represented by value R₂.

    N.sub.m -R.sub.2                                           Equation 5

Register R3 83 contains the number of measured gap durations N_(g), asrepresented in equation 6.

    N.sub.g -R.sub.3                                           Equation 6

Equations 7 and 8 below illustrate the programmed control 16 leastsquares calculations for obtaining optimum laser power level. Equation 7shows finding a value delta Δ (usually non-zero since there is only oneoptimum laser power for each laser power level) that represents thedifference or delta between an average mark duration and an average gapduration. Equation 7 is executed for each calibration sector. A leastsquares analysis of the Equation 7 sector results identifies an optimumlaser power. Such analysis may require an interpolation between twosector delta values to obtain a true optimum laser power. Equation 8sets forth calculating an average clock period for each sector based onmeasured gap and mark durations, i.e., derived from the read calibrationpatterns. ##EQU5##

A clock period T_(c) is calculated at shown in Equation 8 that is basedupon the read calibration pattern from the sector. Constants C₁, C₂ andC₃ are respectively defined in Equations 9-11. ##EQU6##

In Equation 8, constants C₄, C₅, and C₆ are defined in Equations 12-14below.

Equations 9, 10 and 11 defining constants C₁ through C₃ are based ondesign choices of desired average mark and gap durations"<mark>_(desired) and <gap>_(desired). Constants C₁ and C₂ are thequotients of the desired mark and gap durations divided by the result ofequation 7 value T_(c) as calculated for each calibration sector.##EQU7##

The equations 12-14 below are all design data defining a calibrationpattern as recorded in each of the calibration sectors.

    C.sub.4 ≡Number of marks in each calibration pattern Equation 12

    C.sub.5 ≡Number of gaps in each calibration pattern  Equation 13

    C.sub.6 ≡Number of clocks in each calibration pattern Equation 14

The number of clocks (clock periods) indicates the number of clockperiods used to record a complete calibration pattern in eachcalibration sector. Each calibration sector measured clock period isdefined by Equation 8. Clock periods indicated by numeral 114 as theyrelate to marks and gaps are best seen in FIG. 3.

Table I below gives an example of applying the above equations forcalibrating a laser write beam to a plurality of write power levels tobe used in writing blocks of recording code patterns. It is to beunderstood that the laser pulse sequences are repeated a plurality oftimes in each recording in a plurality of sectors store separate copiesof the calibration pattern. Each of the sectors receiving a calibrationpattern is termed a calibration sector. Each calibration sector stores acalibration pattern consisting of a plurality of the below-listed laserpulse sequences for the respective laser power levels P4, P2 and P3. Therecording power level is varied from calibration sector to calibrationsector for having a plurality of laser power levels from which to findan optimum laser power level for each laser power level P4, P2 and P3.

                                      TABLE I                                     __________________________________________________________________________    LASER CALIBRATION                                                                            LASER PULSE  CONSTANTS                                         POWER PATTERN  SEQUENCE     C.sub.1                                                                          C.sub.2                                                                          C.sub.3                                                                           C.sub.4                                                                          C.sub.5                                                                          C.sub.6                           __________________________________________________________________________    P.sub.4                                                                             5G,4M,5G,2M                                                                            11111414T111114T                                                                           5  3  15  2  2  16                                P.sub.2                                                                             5G,4M,5G,3M                                                                            11111414T1111142T                                                                          5  3.5                                                                              17.5                                                                              2  2  17                                P.sub.3                                                                             5G,4M,5G,8M                                                                            11111414T111114131314T                                                                     5  6  30  2  2  22                                __________________________________________________________________________

In the TABLE I column "CALIBRATION PATTERN", G indicates a gap pulse, Mindicates a mark pulse and T indicates laser off for one pulse. Incalibration pattern for P₄, 5G indicates five clock periods, hence fivepulses to indicate a gap while 4M indicates four clock periods toindicate a mark. In the column "LASER PULSE SEQUENCE, numerals 1, 2, 3and 4 respectively indicate laser drive power levels P₁ P₂ P₃ and P₄ andT indicate that no later-described laser writing pulse is emitted. Powerlevel P₁ is a pre-heat power level that provides the pre-heatingdescribed by Hurst, Jr. in said 08/342,196, i.e., creates a gap 45 (FIG.3). The constants C₁ through C₆ are those values calculated usingequations 9 through 14 above. Power level P1 is not calibrated. Thecalibration pattern and the laser pulse sequence are identicalrespectively for the three power calibrateable power levels for creatinga mark as described by Hurst, Jr., supra.

Referring next to FIG. 3, a PWM signal includes a series of alternatingmarks 43 and gaps 45 disposed in successive constant duration data cellsindicated by numeral 110. In an unbanded optical disk rotated at aconstant angular velocity, the physical lengths of data cells 110 varywith the optical disk radius. In a so-called radially-banded opticaldisk rotated at a constant angular velocity, the variation of physicallengths of data can be minimized to be negligible. In optical disksrotated at a constant lineal velocity (as for video and audio disks),the data lengths are constant. The leading edge of each data cell is aleading mark transition 112A, also termed P herein. A trailing edgetransition 113, also termed N herein, identifies a transition betweeneach mark 43 and gap 45 within a data cell 110. Each mark 43 durationindicates plural-bit data while each gap 45 fills out a data cell. Asexplained in Belser et al, a pulsed laser beam creates each mark 43 as aseries of overlapping laser pulses represented in FIG. 3 by hash marks114. The three calibrated write levels P2, P3 and P4. Laser power levelP1, used for creating gaps 45 by not recording any signal on the opticaldisk, is not calibrated. Marks 43 are created by a specific series ofoverlapped pulses, each pulse having one of power levels P2, P3 or P4 ina sequence for making transitions defining marks 43 to be more reliablymachine sensible. The Table I illustrated calibration patterns are anexample of such pulse power level modulation for mark generation. Datacells 110 are shown as having constant durations, no limitation theretointended.

FIG. 4 is a flow chart showing the inventive calibration routines120-122. The three routines 120-122 respectively calibrate power levelsP4, P2 and P3, in that sequence. Each of the routines 120-122 have threesubroutines. Subroutine 1 records a respective Table I definedcalibration pattern in a plurality of optical disk sectors, hereintermed calibration sectors. Each calibration sector receives acalibration pattern at respective diverse laser powers of the powerlevel being calibrated. Other power levels in the illustratedcalibration patterns are at a constant power in all sectors. Thissubroutine uses parameter data identified in Equations 9 through 14 thatdefine calibration pattern design constants C₁ through C₆. Such valuesare design choices, preferably empirically determined, for implementingthe present invention. A first routine 120 calibrates a highest powerlevel P4. Routines 121 and 122 use the calibrated power level P4 whilecalibrating lower power levels P2 and P3 using calibration patternsshown in Table I. Then second subroutine 2 reads the recordedcalibration patterns, sector by sector. The read patterns of the diversesectors are separately analyzed as set forth above with respect to FIG.2 to produce calibration data. The calibration data are stored such thatdata from each sector can be separately analyzed. The second subroutineprepares some parameter data for use in a third subroutine. Finally,third subroutine 3 analyzes the data, sector by sector, for finding anoptimum laser power level of the power level being calibrated. Equation7, supra, defines the desired optimum laser power. The Equation 7results of each sector are compared for finding parameters in Equation 7that results in a zero (0) delta (Δ) value--an optimum laser recordingpower. Later-described FIG. 5 illustrates a sequence of machine stepsthat are used in the illustrative embodiment for illustrating routines120-122.

In routine 120, first subroutine 125 records the Table I indicatedcalibration pattern for laser power P4, the high laser power level usedin the illustrative embodiment. The calibration pattern recording powerlevel for P4 power is linearly varied from sector to sector in aplurality of optical disk sectors. Each sector is recorded at arespective constant laser power for P4. All other power levels are at aconstant laser power for all sectors. Second subroutine 127 reads therecorded P4 calibration patterns, sector by sector. The read back valuesfor each sector are separately stored in programmed control 16 in ausual random access memory (not shown). Third subroutine 129 analyzesthe data stored in programmed control 16 to find the optimum laser powerfor laser power level P4.

Calibrating routines 121 and 122 are identical excepting that routine121 calibrates laser power level P2 (second laser power level to becalibrated) while routine 122 calibrates laser power level P3 (thirdlaser power level to be calibrated). Calibrated write power level P4 isused in routines 121 and 122 for calibrating power levels P3 and P2.First subroutine 131 in routine 121 records the P2 calibration patternof Table I. Subroutine 133 reads the recorded P2 calibration pattern.The calibration data generated from the P2 calibration pattern read fromeach sector is stored in programmed control 16. Subroutine 135,identical to subroutine 129, analyzes the stored calibration data forfinding optimum laser power level for P2.

Routine 122 calibrates laser power P3 in subroutines 137-139.Subroutines 137-139 are identical to steps 131, 133 and 135,respectively, excepting that power level P3 is calibrated.

FIG. 5 illustrates reading and analyzing recorded Table I illustratedcalibration data. FIG. 5 assumes that the calibration patterns have beenrecorded in first subroutines 125, 131 and 137 using the Hurst, Jr.,supra, recording technique. The FIG. 5 illustrated first and secondsubroutines 150 and 151 respectively illustrate details of secondsubroutines 127, 133 and 138 and third subroutines 129, 135 and 139. Theflow chart illustrated process uses circuits illustrated in FIG. 2 andprogramming represented by the flow charts of FIGS. 4 and 5 in programstorage 18 (FIG. 1).

First subroutine 150 begins in step 155 with programmed control 16resetting, via line 84, registers 80-83. Then pattern reading loop 160reads all sectors recorded for calibrating one laser power level. Step162 in pattern reading loop 160 solves some of the equations definingthe illustrated embodiment for preparing parameter data for thirdsubroutine 151. In pattern reading loop 160, step 158, executed in edgedetector 48 (FIG. 2), receives and detects a first recorded mark leadingtransition 112A recorded in a sector being read. In Table I the leftmostP4 power level (indicated in Table I by numeral 4) represents an onsetof the first leading transition in the respective calibration patterns.Step 164 receives and detects each successive signal transition in thecalibration sector being read. Repetitions of step 164 are executed inedge detector 48. Step 166, executed in mark-gap calculator 62,determines whether the processed transition indicates a trailingtransition or edge of a mark 43 (M) or gap 45 (G). As seen in FIG. 3, apositive going transition represents a trailing end of a gap 45 while anegative going transition represents a trailing end of a mark 43.Responding to a detected end of a mark, step 168, executed in mark-gapcalculator 62, measures the duration or width of a mark 43. Step 170,solving Equation 3, accumulates the measured mark durations intoregister R0 80. Step 172 solves Equation 5 to tally the number ofdetected marks in register R2 82. Similarly for gaps 45 (G in step 166)steps 174, 176 and 178 respectively perform the above-described machineoperations set forth in steps 168, 170 and 172 solving Equations 4 and6. From steps 172 and 178, "sector read ?" decision step 182 determineswhether reading the current calibration sector has completed. If thecurrent calibration sector has not been completely read (no), then steps164-178 are repeated until the current calibration sector has beencompletely read. When the current calibration sector has been completelyread (yes), then steps 162 calculate parameter data for the currentcalibration sector. The laser beam continues to scan the spiral tracktoward a next calibration sector to be read (if any).

Step 186 reads the data from the four registers R0-R3 80-83 throughserializer 98 to programmed control 16. Program control 16 then executessteps 190-192 for solving sector related ones of the equations set forthabove. Step 190 solves equation 1 to calculate average mark duration inthe sector. Step 190 solves equation 2 to calculate average gap durationin the sector. Step 192 solves Equation 8 for determining parameterclock period T_(c) that is measured from the data based upon the averagemark and gap durations <mark> and <gap>. Once step 192 solves Equation8, then equations 9-11 are solved. It is pointed out that steps 162 for"intermediate" laser powers P2 or P3, steps 162 may be multi tasked withpreparing a next calibration, as by recording a calibration pattern foranother write power level.

"All sectors read" decision step 196, executed in programmed control 16,checks to see whether or not all of the calibration sectors have beenread. If not (no), steps 158-192 are repeated while scanning successiveones of the calibration sectors. If time to reach the next calibrationsector permits, step 162 can be performed before a next calibrationsector is scanned. If "all sectors read" decision step 196 finds thatall of the sectors have been read (yes), then third subroutine 151 isexecuted. At this time, all of the parameter data for all of thecalibration sectors used to calibrate one laser power level are storedin programmed control 16.

Step 200 in third subroutine 151 solves Equation 7 to develop a deltavalue for each sector, hence each laser power level used for the powerlevel being calibrated. In routines 121 and 122, the previouslycalibrated power level is held constant in all the routine 121 and 122calibration sectors. Generally all of the delta values for each of thecalibration sectors are non-zero, i.e., Equation 7 indicates that theread calibration sector indicates that the laser power used to write(record) a calibration pattern in that sector is not the desired optimumlaser power level. Remember that the optimum laser power for the powerbeing calibrated is indicated by the Equation 7 delta value being zero.Step 201 examines the stored delta values. Step 201, based on theexamination selects a subset of values corresponding to a particularsubset of test laser powers. The selected subset of values will be usedin a linear regression analysis to determine the optimum laser power;i.e. that power where Equation (7) is satisfied and &Delta (Δ)=0. Theselected subset should be chosen over a range wherein Δ depends linearlyon laser power. As the dependence of Δ on laser power is usually verylinear over a wide range of powers this choice is not very critical. Therange can be verified by noting the correlation coefficient of thelinear regression. The center point of the selected subset is found instep 204 by finding any zero value of Δ or two values that are closestto zero, one positive value and one negative value. Subsequently, alinear regression is performed on the selected subset of values forcalculating an x-intercept. This x-intercept represents the laser powerwhere Δ is zero and is chosen as the optimum (calibrated) laser power.Then step 206 indicates the x-intercept is selected as the optimum powerfor the power level being calibrated.

Upon completion of the FIG. 4 illustrated flow chart, optimum laserpowers have been identified for writing power levels P4, P2 and P3.These values are stored within a retentive memory (not shown) inprogrammed control 16 for later use in writing to a laser disk used inthe calibration processing. Each optical disk results in a separate setof optimum laser write power levels.

Since calibration of the intermediate laser power levels P3 and P2 usecalibrated laser power level P4, a constant ratio exists between thethree calibrated laser power levels. Such constant ratio enables latercalibration of one of the laser power levels and extrapolating thecalibration to the other laser power levels without re-calibrating allof the laser power levels.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method of calibrating a laser beam generatingapparatus for supplying any one of a plurality of laser beams having anyone of a plurality of laser beam power levels, including stepsof:generating a first calibration pulse pattern that includes a maximumlaser power level in a predetermined sequence of power levels; applyingsaid first calibration pulse pattern to said laser beam generatingapparatus for producing a first laser beam calibration pulse pattern;analyzing said first laser beam calibration pulse pattern fordetermining a maximum optimum laser beam power level; generating asecond calibration pattern including said optimum laser beam power leveland a second laser beam power level that has a laser beam power levelless than said maximum laser power level; applying said secondcalibration pattern to said laser beam generating apparatus forproducing a second laser beam pulse calibration pattern; and analyzingsaid second laser beam calibration pulse pattern for determining asecond optimum laser power level.
 2. In the method set forth in claim 1,including a step of:selecting said calibration patterns to representpredetermined pulse-width-modulated signals, respectively.
 3. In themethod set forth in claim 1, including steps of:generating a thirdcalibration pattern including said optimum laser beam power level and athird laser beam power level that has a laser beam power level less thansaid maximum laser power level; applying said third calibration patternto said laser beam generating apparatus for producing a third laser beampulse calibration pattern; analyzing said third laser beam calibrationpulse pattern for determining a third optimum laser power level: andmaking power ratios between said first, second and third optimum powerlevels to be a predetermined constant power level difference.